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The TARDIS is going underground. When the Doctor and Romana find themselves buried beneath the surface of an alien world, they're soon swallowed up by a giant burrowing machine. This is where the inhabitants of this planet live - in huge, constantly moving, Drill-towns, chewing up the fuel and resources of the planet in order to survive.

But something else lurks in the earth. Something that feeds on the Drill-towns. Something that is relentless and will not stop.

Although not specifically stated, the idea of the Silex, advanced cyborg-creatures modifying other people to become more Silex in order to be immortal, is reminiscent of the Cybermen, and could possibly be an example of their parallel evolution. Similarly, the group-consciousness Silex making Lucretia into their Queen is similar to the Borg from Star Trek: The Next Generation.